In a new study, scientists from the Max Planck Institute for Marine Microbiology, the Alfred Wegener Institute and the University of Vienna shed light on an unexpected partnership: A marine diatom and a bacterium that can account for a large share of nitrogen fixation in vast regions of the ocean. This symbiosis likely plays a key role for global marine nitrogen fixation and productivity, and thus uptake of carbon dioxide. The newly-discovered bacterial symbiont is closely related to the nitrogen-fixing Rhizobia which live in partnership with many crop plants and may also open up new avenues for engineering nitrogen-fixing plants. The results were published in the current print edition of the journal Nature.
Nitrogen is an essential component of all living organisms. It is also the key element controlling the growth of crops on land, as well as the microscopic oceanic plants that produce half the oxygen on our planet. Atmospheric nitrogen gas is by far the largest pool of nitrogen, but plants cannot transform it into a usable form. Instead, some crop plants like soybeans, peas and alfalfa (collectively known as legumes) have acquired Rhizobial bacterial partners that "fix" atmospheric nitrogen into ammonium, which can be used by plants. This partnership makes legumes one of the most important sources of proteins in food production.
Yet, how marine plants obtain the nitrogen they need to grow has not yet been fully clarified. Scientists from the Max Planck Institute for Marine Microbiology, the Alfred Wegener Institute and the University of Vienna now report that Rhizobia can also form similar partnerships with tiny marine plants called diatoms -- a discovery that solves a long-standing marine mystery and which has potentially far-reaching agricultural applications.
An enigmatic marine nitrogen fixer hiding within a diatom
For many years it was assumed that most nitrogen fixation in the oceans was carried out by photosynthetic organisms called cyanobacteria. However, in vast regions of the ocean there are not enough cyanobacteria to account for measured nitrogen fixation. Thus, many scientists hypothesized that non-cyanobacterial microorganisms must be responsible for the "missing" nitrogen fixation. "For years, we have been finding gene fragments encoding the nitrogen-fixing nitrogenase enzyme, which appeared to belong to one particular non-cyanobacterial nitrogen fixer," says Marcel Kuypers, lead author on the study. "But, we couldn't work out precisely who the enigmatic organism was and therefore had no idea whether it was important for nitrogen fixation."
In 2020, the scientists travelled from Bremen to the tropical North Atlantic to join an expedition involving two German research vessels. They collected hundreds of liters of seawater from the region, in which a large part of global marine nitrogen fixation takes place, hoping to both identify and quantify the importance of the mysterious nitrogen fixer. It took them the next three years to finally puzzle together its genome. "It was a long and painstaking piece of detective work but ultimately, the genome solved many mysteries," says Bernhard Tschitschko, first author of the study and bioinformatician now working at the University of Innsbruck. Co-author and bioinformatician Daan Speth now working at the University of Vienna adds: "Based on the nitrogenase gene fragment we had seen in many marine samples before, one would have expected to find this gene in a Vibrio-related organism, but by carefully piecing together the genetic information it turned out that instead, it belonged to a genome closely related to known Rhizobia, which typically live in symbiosis with legume plants." Together with its surprisingly small genome, this raised the possibility that the marine Rhizobia might be a symbiont.
Source: ScienceDaily
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